The present invention relates to a color image forming apparatus such as a laser beam printer or an electrophotographic copying machine, and to a process cartridge detachably mountable to a main assembly of the apparatus, and more particularly to a color image forming apparatus of a cleanerless type provided with a plurality of developing devices and a process cartridge therefor. Heretofore, the image forming apparatus such as a copying machine, laser beam printer or the like of an electrophotographic type, an image is formed through a series of image forming process generally including a charging process of electrically charging an image bearing member such as a photosensitive drum to a uniform potential; an exposure process for writing an electrostatic latent image by exposing the charged image bearing member; a developing process of developing the electrostatic latent image into a toner image by depositing the toner to the electrostatic latent image; a fixing process of fixing the toner image on the transfer material; a cleaning process of removing residual matter such an untransferred toner or the like on the surface of the image bearing member after the toner image transfer; and so on.
The untransferred toner collected by the cleaning device in the cleaning process is contained in a container, and are then discarded.
Recently, a cleanerless process has been proposed in which the cleaning device is omitted so that image forming apparatus is downsized, and the necessity for the maintenance operation of discarding the untransferred toner is omitted. One example of such cleanerless process apparatus is such that in a developing device, the residual matter remaining on the non-exposed portion is collected, simultaneously with the toner being deposited on the portion having an exposed and therefore attenuated surface potential portion of the image bearing member through reverse development.
The said residual matter not transferred but remaining on the image bearing member even after the transfer step, is collected into the developing device by the electrostatic force provided by a difference (back contrast) between the surface potential of the image bearing member and t developing bias applied to the developer carrying member, after the residual matter passed through the charging step. This occurs in a cleanerless process in a monochromatic laser beam printer.
More particularly, in the case of a one component developer of a negatively chargeable type, the image bearing member is charged uniformly to xe2x88x92600V in the charging step: in t exposure step, such a portion of the image bearing member as corresponds to t image portion is exposed to light, such that surface potential is attenuated so as to provide xe2x88x92200V of the exposed portion; in the developing process, a DC voltage of xe2x88x92400V is applied to the developer carrying member as a developing bias. By doing so, the residual toner charged to the negative polarity remains on the image bearing member as it is in the exposed portion corresponding to the image portion, but in the non-exposed portion, it is collected back into the developing device by the back contrast. Thus, even without the cleaning device, the untransferred toner can be collected, and it reused as the developer.
In the recent laser bean printer market, the color laser beam printer is becoming dominant.
What is desired is inexpensive and small size color printers, as with the case of the monochromatic printers. Therefore, it is desirable to provide a simple electrophotographic system.
However, when the cleaner-less system used in the prior-art monochromatic electrophotographic system is incorporated in the full-color electrophotographic system, the following problems arise:
FIG. 1 is a schematic sectional view of a conventional image forming apparatus. In the case of the above-described negatively chargeable toner, in the transfer step of transferring the toner onto the transfer material, the positive electric charge is applied to the transfer of detail, and therefore, the toner charged to the negative polarity is transferred onto the transfer material. Therefore, most of the residual toner is normally charged to the positive polarity, but after it passes through the charging step, it is charged to the negative polarity, and therefore, remains on the photosensitive drum.
In this manner, the negative charge toner remaining on the image portion of the photosensitive drum is effective to develop the image portion, whereas the negative charge toner remaining in the non-image portion is collected in a developing device. No problem arises in the monochromatic cleanerless, because the toner remaining in the image portion has the same color.
However, in a color electrophotographic system, a plurality of colors of toner are used, and therefore, there is a possibility of resulting in coloring difference due to the untransferred toner.
FIG. 2 illustrates a behavior of the untransferred toner in an image forming station for a second color in a color electrophotographic system.
In t color electrophotographic system, toner images formed on a photosensitive drum in the respective image forming stations are sequentially transferred and superposed on the same transfer material (paper, OHP sheet or the like) fed by transfer material feeding means in the form of a belt.
With this structure of the image forming apparatus, the toner transferred onto t transfer material in the previous station may be deposited onto the photosensitive drum in the current station when the toner reaches the current station (so called xe2x80x9cre-transferxe2x80x9d. If this occurs, the toner is mixed into the untransferred toner in the current station (in FIG. 2, the toner of the positive polarity on the is drum after the image transfer.
The untransferred toner of the previous station (re-transferred toner) is of course different in color. These toner particles are passed through the charging device by which toner particles are charged to the negative polarity, and the toner remaining in the non-image portion is collected in the developing device. However, the re-transferred toner having the different color and remaining in the image portion on the photosensitive drum is not collected by the developing zone and remains in the image portion, with the result of ghost image having a different coloring.
In order to avoid the ghost image resulting from the re-transferred toner, the re-transferred toner in the image portion is prevented in the process upstream of the developing process. In a known example, an auxiliary member in the form of a brush for temporarily collecting the untransferred toner on the photosensitive drum upstream of the charging roller. However, this would result in the complicated process and therefore expensive process.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus and a process cartridge in which color mixture in the developing operation is effectively prevented.
It is another object of the present invention to provide an image forming apparatus and a process cartridge in which re-transferred ghost image is effectively prevented.
It is a further object of the present invention to provide an image forming apparatus and a process cartridge which is suitable to a cleanerless type.
It is a further object of the present invention to provide an image forming apparatus and a process cartridge in which the residual developer on the image bearing member is substantially completely removed during the developing operation.
It is a further object of the present invention to provide an image forming apparatus and a process cartridge in which the polarity of substantially all of the residual developer on the image bearing member is made to be the regular polarity to promote collection during the developing operation.
According to one aspect, the present invention, which achieves at least one of these objectives, relates to an image forming apparatus comprising transferring means for transferring, after a first color developer image is transferred onto an image receiving material in a first image forming station, a second color developer image onto the image receiving material in a second image forming station. The first image forming station includes a first image bearing member, first charging means for electrically charging the first image bearing member, and a first developer carrying member configured and positioned to carry the first color developer to develop an electrostatic image formed on the first image bearing member with the first color developer. The second image forming station includes a second image bearing member, second charging means for electrically charging the second image bearing member, and a second developer carrying member configured and positioned to carry the second color developer to develop an electrostatic image formed on the second image bearing member with the second color developer. The second developer carrying member is capable of collecting a residual developer from the second image bearing member simultaneously with the performing of a developing operation therewith. The second color developer carried on the second developer carrying member is contacted to the second image bearing member. The moving direction of the second developer carrying member is opposite to a moving direction of the second image bearing member at a development position. The second charging means includes a second charging member contactable to the second image bearing member, and the second charging member triboelectrically charges the residual developer on the second image bearing member to a regular polarity.
According to another aspect, the present invention, which achieves at least one of these objectives, relates to a process cartridge detachably mountable to a main assembly of an image forming apparatus. The main assembly comprises transferring means for transferring, after a first color developer image is transferred onto an image receiving material in a first image forming station, a second color developer image onto the image receiving material in a second image forming station. The process cartridge is detachably mountable to the second image forming station. The process cartridge comprises an image bearing member, charging means for electrically charging the image bearing member, and a developer carrying member configured and positioned to carry a second color developer and to develop an electrostatic image formed on the image bearing member with a second color developer. The developer carrying member is capable of collecting a residual developer from the image bearing member simultaneously with the performing of a developing operation therewith. The second color developer carried on the developer carrying member is contacted to the image bearing member. A moving direction of the developer carrying member is opposite to a moving direction of the image bearing member at a development position. The charging means includes a charging member contactable to the image bearing member, and the charging member triboelectrically charges the residual developer on the image bearing member to a regular polarity.
According to still another aspect, the present invention, which achieves at least one of these objectives, relates to an image forming apparatus comprising transferring means for transferring, after a first color developer image is transferred onto an image receiving material in a first image forming station, a second color developer image onto the image receiving material in a second image forming station. The first image forming station includes a first image bearing member, first charging means for electrically charging the first image bearing member, and a first developer carrying member configured and positioned to carry the first color developer to develop an electrostatic image formed on the first image bearing member with the first color developer. The second image forming station includes a second image bearing member, second charging means for electrically charging the second image bearing member, and a second developer carrying member configured and positioned to carry the second color developer to develop an electrostatic image formed on the second image bearing member with the second color developer. The second developer carrying member is capable of collecting a residual developer from the second image bearing member simultaneously with the performing of a developing operation therewith. The second color developer carried on the second developer carrying member is contacted to the second image bearing member.
According to still another aspect, the present invention, which achieves at least one of these objectives, relates to a process cartridge detachably mountable to a main assembly of an image forming apparatus. The main assembly comprises transferring means for transferring, after a first color developer image is transferred onto an image receiving material in a first image forming station, a second color developer image onto the image receiving material in a second image forming station. The process cartridge is detachably mountable to the second image forming station. The process cartridge comprises an image bearing member, charging means for electrically charging the image bearing member, a developer carrying member configured and positioned to carry a second color developer and to develop an electrostatic image formed on the image bearing member with a seconds color developer. The developer carrying member is capable of collecting a residual developer from the image bearing member simultaneously with the performing of a developing operation therewith. The second color developer carried on the developer carrying member is contacted to the image bearing member. A moving direction of developer carrying member is opposite to a moving direction of the image bearing member at a development position.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.